The present invention relates to a process for isomerizing (or hydroisomerizing) normal paraffins, particularly normal paraffins having 4, 5, 6 or 7 atoms of carbon per molecule, in the presence of at least one catalyst based on a particular omega zeolite (or mazzite).
The isomerization of normal paraffins, notably with a low molecular weight, is of considerable importance in the oil industry, in view of the particularly high octane number of isoparaffins.
It is therefore interesting to be able to convert C.sub.4 -C.sub.7 and especially, C.sub.5 -C.sub.6 -n-paraffins into isoparaffins in order to obtain a fuel with a high octane number. This process allows improving light gasoline fractions and particularly straight-run head fractions.
There are mainly three different types of processes for the isomerization of normal paraffins:
"low temperature" processes (about 20.degree. to 130.degree. C.), utilizing a catalyst of the Friedel-Crafts type, such as aluminum chloride,
"average temperature" processes (about 150.degree. C.), utilizing as a catalyst a supported metal such as platinum on halogenated alumina, and
"high temperature" processes (at least 250.degree. C.), utilizing zeolitic supports generally associated with a hydrogenating metal from group VIII of the periodic table of elements.
The thermodynamic equilibrium is more favorable to isoparaffins at low temperature, one advantage of the first two types of processes. These processes nevertheless have major drawbacks: catalysts of the Friedel-Crafts type and platinum on halogenated alumina catalysts, because of their corrosive nature, require very costly reactors made of special alloys and they are, moreover, very sensitive to water and to traces of sulfur.
"High temperature" processes have been the subject of numerous patents for about twenty years. Most catalysts that are described therein are made of zeolite, generally mordenite, in the acid form, with or without a hydrogenation promoter.
Mordenite-based catalysts obtained with the following treatments have thus been claimed:
exchange of Na.sup.+ ions by NH.sub.4.sup.+ or H.sup.+ ions (U.S. Pat. No. 3,190,939),
acid treatment:
hot acid--NH.sub.4.sup.+ succession (U.S. Pat. No. 3,442,794) PA1 hot acid--cold acid succession (U.S. Pat. No. 3,475,345) PA1 acid solution containing Na.sup.+ or K.sup.+ ions U.S. Pat. Nos. 2,272,737, 4,359,409 and 4,400,576),
thermal treatments under controlled humidity (U.S. Pat. Nos. 2,181,928, 3,836,597, and 3,842,114).
In the same vein, other patents relate to processes for dealuminizing mordenite:
severe acid treatments: HCl 12N, 100.degree. C. (U.S. Pat. No. 3,480,539),
self-steaming (calcining in a cured atmosphere) between 430.degree. and 820.degree. C., followed by acid attack (U.S. Pat. No. 3,506,400 and 3,551,353).
The use, for the isomerization of normal paraffins, of a mordenite with a particular morphology, modified by a succession of thermal treatments and acid treatments, is also described in U.S. Pat. No. 4,727,217.